Electronic flash device

ABSTRACT

The electronic flash device includes a flash tube, a light sensitive electronic component for converting light reflected from an object to be illuminated into a current proportional to the quantity of light reflected, an integrating stage adapted to integrate this current, an a flash duration limiting circuit. On the occurrence of a pre-set voltage at the integration stage, the duration limiting circuit terminates the discharge of light in the flash tube. An operating voltage is applied to the light sensitive electronic component simultaneously with the commencement of ignition in the flash tube. An electronic switching device is connected with the light sensitive component and adapted to cut out disturbing charging and discharging currents associated with the inherent capacitance of the light sensitive electronic component and occurring upon a sudden application of the operating voltage. The time in which the switching device remains active corresponds at least to the charging and discharging duration of the inherent capacitance of the light sensitive component; the switching device is deactivated before the light emission of the flash tube begins.

ted States Patent [191 Pecher et a1.

[ Oct. 30, 1973 ELECTRONIC FLASH DEVICE [76] Inventors: Wolfgang Pecher, 8550 J. F.

Kennedy Ring 20, Forchheim; Jiiergen Frlsch, 8S1 Kasersh. 47, Fuerth, both of Germany 22 Filed: May 17, 1972 211 App]. No.: 254,308

Related US. Application Data [63] Continuation-impart of Ser. No, 66,783, Aug. 25,

1970, abandoned.

Primary Examiner-Roy Lake Assistant Examiner-Lawrence J. Dahl Attorney-McDougall, Hersh & Scott [5 7] ABSTRACT The electronic flash device includes a flash tube, a light sensitive electronic component for converting light reflected from an object to be illuminated into a current proportional to the quantity of light reflected, an integrating stage adapted to integrate this current, an a flash duration limiting circuit. On the occurrence of a pre-set voltage at the integration stage, the duration limiting circuit terminates the discharge of light in the flash tube. An operating voltage is applied to the light sensitive electronic component simultaneously with the commencement of ignition in the flash tube. An electronic switching device is connected with the light sensitive component and adapted to cut out disturbing charging and discharging currents associated with the inherent capacitance of the light sensitive electronic component and occurring upon a sudden application of the operating voltage. The time in which the switching device remains active corresponds at least to the charging and discharging duration of the inherent capacitance of the light sensitive component; the switching device is de-activated before the light emission of the flash tube begins.

11 Claims, 4 Drawing Figures l" 1 l 34% l 31 ll I v 1 ll 39 H :37 l 317 c I PATENTED HUI 30 I975 SHEET 10F 2 Fire: 1

SHEET 2 BF 2 PATENTED MIT 30 I975 ll jl ELECTRONIC FLASH DEVICE This application is a continuation-in-part of application Ser. No. 66,783, filed Aug. 25, 1970, and now abandoned.

BACKGROUND 'OF THE INVENTION 1. Field of the Invention The invention relates to photographic electronic flash devices equipped with automatic means for controlling the quantity of light emitted. More specifically, the invention relates to such devices with a control means comprising a light sensitive electronic component which converts the light reflected from the object to be illuminated into a current proportional to the quantity of light reflected, an integrating stage which integrates this current, and a flash duration limiting circuit. When a pre-set voltage appears at the integration stage in response to the sensing of a quantity of light which can be pre-set, the discharge of light from the flash tube is terminated. In order to provide protection against spurious effects due to other sources of light, the operating voltage in these devices is applied to the light sensitive electronic component simultaneously with the ignition in the flash device.

2. The Prior Art Prior-art electronic flash devices may be represented by the following U.S. Pat. Nos.: Elliott Re26,999; Hoffer 3,517,255; Ogawa 3,519,879; Ackerman 3,541,386, and Krusche 3,585,442. In these devices, the light emitted after the initiation of the flash is reflected by the object to be photographed. Via the lenses of the camera, the reflected light exposes the film in the camera; simultaneously, the reflected light is received by a light sensitive electronic component and is converted into a corresponding current.

This current is integrated in a capacitor so that the voltage of the capacitor is a measure of the quantity of the reflected light. When the voltage reaches a value, which can be pre-established and which corresponds to the optimum exposure of the film, a flash duration limiting device is actuated thereby terminating further emission of light.

If the light measuring and flash limiting circuit in such a device is immediately ready for actuation upon firing of the associated flash device, light from a source other than such flash device (this other light may be many times stronger than the light reflected from the object to be photographed), can actuate the flash limiting means prematurely or can exert a spurious effect on the light measuring means to cause a false measurement of the reflected light so that an incorrect exposure of the film in the camera results.

Avoidance of difficulties with light from other flash sources can be ensured by blocking the light measuring and flash limiting means before closing of the contacts which bring about ignition of the flash tube. This blocking out of the effect of a foreign source of flash light is disclosed and claimed in Muller application, Ser. No. 787,988, filed Dec. 30, 1968, and assigned the assignee of the present application.

Another technique for preventing spurious effects of flashes from extraneous sources resides in applying the operating voltage to the light measuring and flash limiting means only during the time in which the flash light is emitted from the associated flash device, the voltage being derived in this case from the ignition operation or from the current pulse of the light emitting flash tube, reference being had to the aforementioned Muller et al. application. It is also possible, as shown in the above-mentioned Elliott patent, to use light emitted from the flash tube of the device for the control of the light sensitive part which actuates the flash limiting device.

There are also arrangements for achieving this blocking in which the light sensitive component is itself used for integration, making use of its own properties: See the 1968 German publication Elektronik, issue No. 5, pages 137 et seq. However, in this case, it is not possible to compensate for switching lags, as referred to in the Muller et al. application.

German patentschrift 1,146,669 describes a blocking arrangement of the type under consideration, in which the light sensitive component is simultaneously switched on, so as to be ready for operation with the initiation of ignition of the flash tube, by mechanically switching on the operating voltage. In this arrangement, the light sensitive component and the integrating stage can be separated from each other by a switch; the integrating stage is bridged over via the base-emitter path of a transistor which can operate as a variable resistance.

In automatic flash devices, of the type referred to herein, a germanium photodiode is generally used as the measuring cell for the light reflected from the object. The germanium photodiode has a good degree of linearity between the strength of illumination and current and possesses a small inherent capacitance. Due to this small inherent capacitance, when the operating voltage is suddenly applied there are practically no disturbing pulses impairing the function of the circuit and there is no false charging of the integrating device. The germanium photodiode has a relatively high dark current and is very dependent on temperature. This relatively large dark current of the germanium photodiode must be balanced in order to achieve satisfactory accuracy of the light measuring and regulating means. This may be achieved, for example, by causing a balancing current to flow via the blocking resistance of a further germanium diode. In this manner it is possible to obtain compensation over a large temperature range. However, in this case it is necessary to pair the germanium photodiodes and the balancing or dummy germanium photodiodes because of the relatively large variations in the dark currents and the blocking currents. This arrangement is disadvantageous because it requires a considerable number of measuring operations.

In the use of silicon photodiodes, the dark currents are generally so low that these devices do not cause any noticeable falsification of the measurement of light. A particular dark current compensation system is therefore not necessary. Unfortunately, silicon diodes of high sensitivity have a relatively high inherent capacitance. Accordingly, upon the sudden application of the pacitance of the transistor in such a manner that this recharging current flows into the base-emitterpath as a control current and a collector emitter current, amplifiedby the current gain which flows to the integrating device. This may result in spurious actuation of the light duration limiting device, or a spurious measurement of light by the false charging of the integration capacitor.

Because of the undesirable effects resulting from the inherent capacitances of the silicon diodes and photo transistors, such components have not previously been used for automatic light control in electronic flash devices of the type which provide against extraneous sources of light by applying the operating voltage to the light measuring device at the time of actuation or at the time of the initiation of emission of light from the device itself.

SUMMARY OF INVENTION A primary object of the present invention is to provide an electronic flash device in which the spurious effects of inherent capacitancies of light sensitive components in the light measuring circuits resulting from the abrupt application of the operating voltage are avoided.

Another object of the invention is to provide an electronic flash device which may advantageously employ semiconductor components with an inherent capacitance.

A further object of the invention is to provide an electronic flash device wherein the light measuring circuit is ready for operation a suitable time before the initiation of light emissions from the flash tube.

In accordance with one aspect of the invention, an electronic flash device of the above-described type is provided in which the light sensitive electronic component is connected with an electronic switching device, the latter being adapted to cut out the disturbing charging and discharging currents of the inherent capacitance of the light sensitive electronic component occurring upon the sudden application of the operating voltage. The electronic switch device is so characterized that the time in which it remains activated corresponds at least to the time of charging and discharging of the inherent capacitance of the light sensitive component but is less than the time between closing of the flash contacts and commencement of light emission of the flash tube.

DESCRIPTION OF THE DRAWINGS Several embodiments of electronic flash light devices in accordance with the invention wiil now be described in detail with reference to the drawings wherein:

FIG. 1 is a block circuit diagram of an electronic flash device in accordance with the invention;

FIG. 2 is a circuit diagram of the embodiment of FIG. 1 in greater detail; I

FIG. 3 is a circuit diagram of another embodiment of the invention; and

FIG. 4 is a circuit diagram of still another embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows in principle the electronic flash device in accordance with the present invention. By a suitable circuit, designated 1 and to be described in detail hereinafter, the operating voltage is applied to the light sensitive element 2 at the time the flash tube is ignited. The inherent capacitance 21 of the element 2 is denoted in broken lines. The circuit includes an integrating capacitor 5 and a compensating or balancing resistor 4 for close-up photography. The integration-compensating portion 4,5 is connected with the flash limiting circuit 6 to be described hereinafter. If the voltage at the input of the circuit 6 exceeds a certain adjustable threshold value, the light emission or the flash tube is abruptly ended by, for example, being short-circuited by the ignition of a low resistance switching tube connected in parallel with it.

An electronic switch 3 is connected with the integrating compensating circuit portion 4,5 and is adapted to close for a short period of time. The actuation of the switch 3 is brought about, for example, by the leading portion of the operating voltage surge from circuit portion 1 or from the front or leading portion of the flash tube current surge.

Upon actuation of the contacts of the flash device on the associated camera, the operating voltage is thus suddenly switched to flow to the light measuring devices 2,4 and 5. Simultaneously with ignition of the flash tube, the electronic switch 3 is closed for a short period of time. Through this closed switch, the charging and discharging current resulting from the inherent capacitance 21 of the light sensitive device 2 is passed so that it cannot influence the integrating compensating circuit portion 4,5. The duration of closing of the switch 3 is so chosen that the charging and discharging of the inherent capacitance during this period is substantially terminated before opening of the switch 3, but such switch is opened again before the light emission of the flash device begins that is to say, before a substantial quantity of light has been reflected by the object to be photographed so that the actual light measurement is not influenced by the switch 3.

As is known to those skilled in the art, high interfering voltage pulses occur in the device due to the high voltage ignition pulse for the flash tube. The switch 3 substantially increases the reliability of the circuit as concerns a false response due to disturbing voltage pulses, since such switch, while it is closed, also shortcircuits the input of the flash limiting circuit 6 so that the latter is immune to disturbing pulses.

A further advantage of the switch 3 is in that while it is closed any residual charge remaining in the integrating capacitor 5 is discharged so that for each light measurement a fully discharged capacitor is always presented and therefore spurious measurements due to residual charges are avoided. This is especially important in the case of devices with short flash sequence times, for example, flash stroboscopes.

The present invention permits the use of light sensitive components with a high inherent capacitance in circuits such as those of FIG. 1. Without the electronic switch 3, the operational voltage current from the cirw cuit portion 1, and the effect of the inherent capacitance of the light sensitive component as explained above, would then produce a voltage surge at the compensating resistor 4, and this surge might cause the flash limiting circuit 6 to be activated. If the compensating resistor 4 is not used in the construction of the integrating circuit, upon switching on of the operating voltage, the charging and discharging of the inherent capacitance brings about undesired charging of the integration capacitor before light is emitted by the flash device.

Referring now to FIG. 2, it will be understood that the circuit portion 1 generates the operating voltage for the light sensitive circuit. These voltages are generated directly from the discharge of a capacitor 29, which capacitor triggers the flash device or tube 31. Capacitor 29 is charged from a battery or an AC-DC converter connected between terminals 32 and 33 via a voltage divider composed of resistors 34 and 35. The discharge of capacitor 29 occurs when a synchronizing switch 36 is actuated, this switch being either in the form of contacts operated by the associated camera shutter or by a switch on the electronic flash device itself. Closing of this switch creates a voltage surge in the primary winding 37 of a triggering transformer 38; this induces a voltage pulse in secondary winding 39 which triggers the flash device 31 and which simultaneously creates, in a tertiary transformer winding 40, a voltage surge which charges a storage capacitor 41, through a rectifier 42, to produce the supply voltage for the light sensitive circuit including the phototransistor 22. Capacitor 41 discharges slowly through a resistor 28 having a resistance such that during the longest possible flash duration, which can be in the order of l msec, the voltage across capacitor 41 remains virtually constant, and such that during the shortest interval between successive firings, which can be in the order of 5 seconds, the capacitor 41 will be totally discharged.

Still referring to the embodiment of FIG. 2, the light produced by flash device 31 and reflected from the object being photographed is received by the phototransistor 22 to vary the current therethrough and hence to charge capacitor 5 at a rate proportional to the reflected light intensity. The voltage across capacitor 5 thus increases with the exposure time and is applied to the base of a silicon controlled rectifier 44. As soon as the voltage across capacitor 5 has reached the pre-set value, the silicon controlled rectifier 44 becomes conductive and fires a flash limiting device or quench tube 45 by discharging a triggering capacitor 46 through a firing transformer 47. The firing of the quench tube 45 creates a short-circuit across the tube 31 and thus abruptly terminates the flash being generated thereby. Capacitor 46 is initially charged via a voltage divider consisting of resistors 48 and 49. The firing voltage of silicon controlled rectifier 44 is adjusted by controlling a potentiometer 50 connected in parallel with a capacitor 51.

For a more complete description of the nature and operation of circuit portions 1 and 6, reference should be had to the aforementioned Muller application.

The switch circuit 3 of the present invention, as shown in FIG. 2, includes a transistor 8, resistors 9 and 10, and a capacitor 11. When the switch 36 is closed thereby causing the voltage abruptly to flow to the phototransistor 22, the voltage surge is transmitted through the capacitor 11 to the resistor 10. A positive control current will flow in the base of transistor 8, the collector-emitter path of the transistor 8 becomes conducting and thus the integrating-compensating circuit portion 4, 5 is short-circuited to ground. In this manner, the intefering current resulting from recharging of the collector-base capacitance of the phototransistor 22 is conducted directly to ground. The capacitor 11 will discharge through resistors 9 and through the baseemitter path of the transistor 8. As soon as the voltage at the resistor 9 exceeds the base threshold voltage of the transistor 8, the collector-emitter path thereof becomes non-conducting. Accordingly, at such time the integrating-compensating circuit portion 4, 5 is again activated for measuring an integrating light sensed by the phototransistor 22. The closure time of the transistor 8 is of course determined by the characteristics or values of the resistors 9,10 and the capacitor 11. The switch 8 remains conducting at least for the time of charging and discharging of the inherent capacitance of the phototransistor 22; the transistor 8 becomes nonconducting prior to the time that light commences to be emitted form the flash device 31.

When a light sensitive device, such as a photodiode or phototransistor is used, a further interfering effect could be generated upon the abrupt application of the operating voltage. Because of the high current pulse, which flows through the phototransistor 22 upon the application of the operating voltage, a recovery time is necessary after the phototransistor is deactivated to allow the same to react normally during the next operation of the electronic flash device. Due to the use of the current limiting resistor 7 connected to the collector terminal of the phototransistor 22, the amplitude of the charging current thereto is reduced. However, the charging and discharging time of the collector-base capacitance of the device 22 is not so extended that the measurement of the reflected light is influenced by this charging and discharging.

Turning now to FIG. 3, which shows another embodiment of the invention, the various elements which correspond to the elements of the embodiment of FIGS. 1 and 2 are indicated by the same reference characters. This embodiment avoids the false effects due to the charging and discharging of the collector-base capacitance of the phototransistor 22 by employing a shortcircuit to ground at the base of the phototransistor. This short-circuit includes a diode l2 actuated by a monostable blocking oscillator, consisting of a transistor 8, a transformer 17, resistors 13, 1S and a capacitor 16. This oscillator is triggered by a capacitor 14 due to the action of the leading portion of the voltage surge applied to the light sensitive portion of the circuit. The closing duration of the diode 12 is determined by the characteristics of the resistors l3, 15, the capacitor 16, and the transmission ratio of the transformer 17. The diode 12 must have a low inherent capacitance and a low blocking current. It will be understood that the embodiment of FIG. 3 operates in essentially the same manner as the embodiment shown in FIGS. 1 and 2 to prevent the false or spurious effects caused by the sudden application of the operating voltage.

Still another embodiment of the invention is shown in FIG. 4. Again, the elements of the FIG. 4 embodiment which correspond to the embodiment of the invention disclosed in FIGS. 1 and 2 are identified by the same reference characters. According to this embodiment, a capacitor 18 is charged positively and a capacitor 19 is charged negatively. The switch circuit 3 of this embodiment includes resistors 20, 23, a capacitor 24, and diodes 25, 26. The negative voltage of the capacitor 19 causes a flow of current through the resistor 20, capacitor 24 and diodes 25 and 26. When the diode 26 becomes conducting, the integrating-compensating circuit portion 4, 5, and also the input to the flash limiting circuit portion 6, becomes short-circuited to ground.

Upon the discharge of the capacitor 24, the current through the diodes 25, 26 decreases until the voltage at point A with respect to ground reaches a zero value and then changes to a positive value. Thereupon, the diodes 25, 26 become non-conducting and the integratingcompensating circuit portion 4, is again conditioned for measuring and integrating the light received at the photo element 27. By selecting the characteristics of the resistors 20, 23 and the capacitor 24, it is of course possible to determine the closing time of the diodes 25, 26. it will be understood that the current through the diode 26 is greater than the charging and discharging current of the inherent capacitance of the photo element 27, and that this results in a rise of positive voltage values at point A at a rate more rapid than the voltage rise in the integrating-compensating circuit portion 4, 5 during measuring and integrating of light.

Therefore, the charging and discharging of the inherent capacitance of the photodiode 27 and the attandant spurious voltage pulses have no effect on the integrating-compensating circuit portion 4, 5 or the flash limiting circuit portion 6, when using the embodiment of the invention shown in FIG. 4.

We claim:

1. In a photographic flash device of the type including an electronic flash element and an automatic light terminating circuit therefor, which circuit includes means for generating an operating voltage and a light sensitive electronic component for converting the light reflected from an object to be photographed into a current proportional to the quantity of such reflected light,

means adapted to integrate said current, the light sensitive electronic component being of the type which, upon a sudden application of a voltage pulse thereto, generates charging and discharging currents due to its inherent capacitance, said circuit further including flash duration limiting means adapted, upon the occurrence of a pre-set voltage appearing in said integration means resulting from activation of said light sensitive electronic component by the reflected light, to terminate the emission from said flash element, wherein the operating voltage is applied abruptly to said light sensitive electronic component simultaneously with ignition of the electronic flash element, the improvement comprising, electronic switching means connected to said circuit means for generating the operating voltage so as to become conductive upon the application of the operating voltage to said light sensitive electronic component, said switching means also being connected in said circuit between said light sensitive electronic component and said integrating means for short-circuiting the latter to ground when the switching means becomes conductive, discharge means connected between said operating voltage generating means and said switching means thereby to render the latter conductive upon application of the operating voltage and non-conductive upon discharge of the discharge means, said switching means being rendered conductive by the operating voltage for a period of time at least as great as the period of time during which said light sensitive electronic component generates charging and discharging currents due to its inherent capacitance, and switching means being rendered non-conductive by the discharge means prior to the time of commencement of light emission from said electronic flash element, whereby said switching means compensates for the charging and discharging currents generated by said light sensitive electronic component due to its inherent capacitance.

2. The improvement according to claim 1 wherein said switching means comprises a transistor.

3. The improvement according to claim 1 wherein said switching means comprises at least one diode.

4. The improvement according to claim 1 further defined by, said switching means including a transistor and connected with said circuit for short-circuiting one electrode of said transistor upon application of the operating voltage to said light sensitive electronic component.

5. The improvement according to claim 4 wherein said transistor is short-circuited through its collectoremitter path.

6. The improvement according to claim 1 wherein said switching means comprises a thyristor connected with said circuit so as to short-circuit said integration means through the anode-cathode path of the thyristor upon the latter becoming conductive in response to the application of the operating voltage to said light sensi tive electronic component.

7. The improvement according to claim 4 further defined by an RC circuit connected with the control electrode of said transistor for short-circuiting the latter upon the application of the operating voltage.

8. The improvement according to claim 1 further defined by, said light sensitive electronic component being a phototransistor, a resistor connected in series with the collector of said phototransistor for limiting the charging current of said phototransistor.

9. The improvement according to claim 1 wherein said light sensitive electronic component is a photodiode.

10. The improvement according to claim 1 wherein said switching means comprises a monostable blocking oscillator.

11. The improvement according to claim 10 wherein said switching means comprises a diode actuated by said oscillator thereby to short-circuit said integration means. 

1. In a photographic flash device of the type including an electronic flash element and an automatic light terminating circuit therefor, which circuit includes means for generating an operating voltage and a light sensitive electronic component for converting the light reflected from an object to be photographed into a current proportional to the quantity of such reflected light, means adapted to integrate said current, the light sensitive electronic component being of the type which, upon a sudden application of a voltage pulse thereto, generates charging and discharging currents due to its inherent capacitance, said circuit further including flash duration limiting means adapted, upon the occurrence of a pre-set voltage appearing in said integration means resulting from activation of said light sensitive electronic component by the reflected light, to terminate the emission from said flash element, wherein the operating voltage is applied abruptly to said light sensitive electronic component simultaneously with ignition of the electronic flash element, the improvement comprising, electronic switching means connected to said circuit means for generating the operating voltage so as to become conductive upon the application of the operating voltage to said light sensitive electronic component, said switching means also being connected in said circuit between said light sensitive electronic component and said integrating means for short-circuiting the latter to ground when the switching means becomes conductive, discharge means connected between said operating voltage generating means and said switching means thereby to render the latter conductive upon application of the operating voltage and non-conductive upon discharge of the discharge means, said switching means being rendered conductive by the operating voltage for a period of time at least as great as the period of time during which said light sensitive electronic component generates charging and discharging currents due to its inherent capacitance, and switching means being rendered non-conductive by the discharge means prior to the time of commencement of light emission from said electronic flash element, whereby said switching means compensates for the charging and discharging currents generated by said light sensitive electronic component due to its inherent capacitance.
 2. The improvement according to claim 1 wherein said switching means comprises a transistor.
 3. The improvement according to claim 1 wherein said switching means comprises at least one diode.
 4. The improvement according to claim 1 further defined by, said switching means including a transistor and connected with said circuit for short-circuiting one electrode of said transistor upon application of the operating voltage to said light sensitive electronic component.
 5. The improvement according to claim 4 wherein said transistor is short-circuited through its collector-emitter path.
 6. The improvement according to claim 1 wherein said switching means comprises a thyristor connected with said circuit so as to short-circuit said integration means through the anode-cathode path of the thyristor upon the latter becoming conductive in response to the application of the operating voltage to said light sensitive electronic component.
 7. The improvement according to claim 4 further defined by an RC circuit connected with the control electrode of said transistor for short-circuiting the latter upon the application of the operating voltage.
 8. The improvement according to claim 1 further defined by, said light sensitive electronic component being a phototransistor, a resistor connected in series with the collector of said phototransistor for limiting the charging current of said phototransistor.
 9. The improvement according to claim 1 wherein said light sensitive electronic component is a photodiode.
 10. The improvement according to claim 1 wherein said switching means comprises a monostable blocking oscillator.
 11. The improvement according to claim 10 wherein said switching means comprises a diode actuated by said oscillator thereby to short-circuit said integration means. 